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major reference

...orbits than are other bodies in the solar system. In general, comets were initially classified into two dynamical groups: the short-period comets with orbital periods shorter than 200 years and the
long-period comets with orbital periods longer than 200 years. The short-period comets were split into two groups, the Jupiter-family comets with periods shorter than about 20 years and the...

cosmic impacts

...way that would make them cross Earth’s orbit. The objects that fall into this category are asteroids and comets in short-period orbits—together called near-Earth objects (NEOs)—and those
long-period comets that make their closest approach to the Sun inside Earth’s orbit. Short-period comets complete their orbits in less than 200 years and so likely have been observed before; they...

study of characteristics

...to predict its return), the comet is named for the original discoverer and also the observer(s) who found it again. A designation of “C/” before a comet’s name denotes that it is a
long-period comet (period greater than 200 years), while “P/” denotes that the comet is periodic; i.e., it returns at regular, predictable intervals of fewer than 200 years. A designation...

In 1995 the IAU implemented a new identification system for each appearance of a comet, whether it is periodic or long-period. The system uses the year of the comet’s discovery, the half-month in the year denoted by a letter A through Y (with I omitted to avoid confusion), and a number signifying the order in which the comet was found within that half-month. Thus, Halley’s Comet is designated...

...calculate orbits improved, it became obvious that most comets were on elliptical orbits and thus were members of the solar system. Many were recognized to be periodic. But some orbit solutions for
long-period comets suggested that they were slightly hyperbolic, suggesting that they came from interstellar space. That problem would not be solved until the 20th century.

...massive planet, was a highly unlikely event and probably could not account for the number of short-period comets then known. Also, no comets had ever been observed on truly hyperbolic orbits. Some
long-period comets did have orbit solutions that were slightly hyperbolic, barely above an eccentricity of 1.0. But a truly hyperbolic comet approaching the solar system with the Sun’s velocity...

...estimated that the gravitational focusing by the Sun would bring the particles together only about 150 AU behind the Sun and solar system. But that did not agree well with the known orbits of
long-period comets, which showed no concentration of comets that would have formed at that distance or in that direction. In addition, the total amount of gases that could be adsorbed on a sandbank...

...them into interstellar space. Van Woerkom also showed that because of Jupiter, repeated passages of comets through the solar system would lead to a uniform distribution in orbital energy for the
long-period comets, with as many
long-period comets ending in very long-period orbits as in very short-period orbits. Finally, van Woerkom showed that Jupiter would eventually eject all the...

Oort noticed that the number of
long-period comets returning to the planetary system was far less than what his model predicted. To account for that, he suggested that the comets were physically lost by disruption (as had happened to Biela’s Comet). Oort proposed two values for the disruption rate of comets on each perihelion passage, 0.3 and 1.9 percent, which both gave reasonably good results...

...only a few hundred-thousandths of the solar gravitational attraction, but that was enough to change the time when the comet would return. Later, Marsden and colleagues computed the rocket forces for
long-period comets and found that there too the mean residuals were reduced. For the
long-period comets, the rocket force was typically a few ten-thousandths of the solar gravitational attraction....

...astronomer Julio Fernández suggested that a comet belt beyond Neptune would be a good source for the short-period comets. Up until that time it was thought that short-period comets were
long-period comets from the Oort cloud that had dynamically evolved to short-period orbits because of planetary perturbations, primarily by Jupiter. But astronomers who tried to simulate that process...

...recognized that a key element in understanding the short-period comets was their relatively low-inclination orbits. Typical short-period comets have orbital inclinations up to about 35°, whereas
long-period comets have completely random orbital inclinations from 0° to 180°. Fernández suggested that the easiest way to produce a low-inclination short-period comet population was...

...as yet unknown, there is no good explanation as to why some areas remain active and others do not. It is known that this is likely an aging effect, as the active fraction on the nucleus is large for long-period and Halley-type comets, which have made relatively few approaches close to the Sun, and very low, typically only a few percent, for short-period, Jupiter-family comets, which have made...